Avaliação de características associadas a termotolerância em vacas da raça Holandês de variedade preta ou vermelha

Abstract

Heat stress is among the main problems that affect dairy farming and occurs when there is an imbalance between heat produced by the body and absorbed from the environment and the dissipation capacity, influenced by the conformation of the layer of hair and skin. The coat colors differ in absorbance, in black spots is 90%, in red spots is 60-70% and in white spots is 40%. Generally, studies that compare thermoregulation in the Holstein breed include mostly black or white and intermediate animals and not the red ones. Thus, the study aimed to compare the thermoregulatory capacity of Holstein cows with red and white or black and white, managed in semiextensive system, under thermoneutrality and heat stress. Holstein cows created in semi-extensive system were allocated in the groups: HRW group, with red and white coats, and HBW group, with black and white coats. For each HRW cow, an equivalent HBW cow was selected for percentage of spots, body condition score, weight, milk production, days in milk, parity and reproductive status. Environmental conditions were registered every hour and the temperature and humidity index (THI) was calculated, considering the absence of stress when the THI < 68. Internal body temperature was recorded every 30 minutes using intravaginal thermometers fixed in new progesterone implants (n=18). The surface and lacrimal caruncle temperatures were obtained through thermographic images (n=26 cold season; n=34 hot season). The sweating rate was estimated using the SCHLEGER and TURNER colorimetric test. In the cold season, environmental conditions of absence of stress were observed throughout the period. Internal body temperature, time in hyperthermia, sweating rate and lacrimal caruncle temperature did not differ between groups. Only the average surface temperature (HBW: 30.9±0.3 ºC; HRW: 29.6±0.3ºC; p=0.02) and maximum (HBW: 32.0±0.3ºC; HRW: 30, 9±0.3ºC; p=0.05) were lower in the HRW group. In the hot season, in the hottest hours of the day, the THI was mild and moderate stress and in few moments was observed severe stress. In the coldest hours, the classification was of absence of stress. Surface, lacrimal caruncle and internal body temperature, time in hyperthermia and sweating rate did not differ between groups. The mean internal body temperature was slightly lower in the HRW group (HBW: 38.75±0.01 ºC; HRW: 38.62±0.1 ºC; p=<0.0001). When the evaluation was segmented into quartiles, there was an effect of the group and the THI (P<0.001), there was no interaction (p=0.62). Thus, under mild to moderate heat stress, red and white Holstein cows did not prove to be more thermotolerant than black and white cows. The statistically significant alterations may not represent physiological advantages to the red animals under the tested conditions.